why does coffee get bitter over time?
i just bought a thermal carraffe coffeemaker (i.e. it has no
continuous "hot plate") to try to solve the problem. i thought my
coffee got bitter over time because it was being scorched by the
burner. yet, i have still noticed a degradation in taste from when
brewing stops (when the coffee tastes its best). i am "enjoying" a cup
right now that was brewed 2 hours ago (still hot in the thermos!) but
is starting to taste shitty.
or it's my imagination?

No, Deerob, it's not your imagination.
You're right that keeping your coffee on the hot plate is a bad idea.
Commercial coffemakers are large, expensive pieces of equipment with
carefully calibrated heat...and even at that, coffeeshops will
generally throw out any coffee older than 20 minutes.
The reason you're finding your coffee different after being in the
thermos is quite different, however. The flavour of coffee comes from
several volatile oils which our taste buds have learned to appreciate.
Heat is one of the enemies of these oils, certainly...coffee heated
too high, or kept on heat too long, will lose most of its flavour and
aroma (or develop an unpleasant scorched flavour). However, there are
other factors at play. The most important of these is oxygen, and
that's what you're up against now.
Oxygen is necessary to our ability to taste coffee. That's why you
get the greatest perception of flavour when you open your mouth to
inhale, after a swallow. As you inhale, oxygen hits the coffee still
on your tastebuds; exhale the coffee-laden breath from your mouth, and
the vapour (including those volatile oils we spoke of) passes over a
large number of sensory centres located in your sinus passages; many
more than the number of your tastebuds. It is in this reaction to
oxygen that the volatile oils create their flavours for us. The place
we want this to happen, though, is within your "tasting apparatus," ie
your mouth and airways.
In an open cup or pot, the volatile oils oxydise quickly. That's why
reheating a cup of cold coffee is such a singularly unpleasant thing.
Your thermal carafe is much better; when you fill it, it is tightly
capped and has only a small airspace. The degradation caused by heat
will be minor; likewise the degradation caused by the oxygen trapped
in your thermos. Your first cup from the thermos (especially if taken
within a half-hour or so) will be almost indistinguishable from fresh.
The longer the coffee is in the thermos, the more of its volatile oils
will be oxydised by the air trapped under the cap. This is especially
true if the thermos is being moved about (say, in a lunchbox) since
this will expose more of the coffee to the oxygen.
Once you've opened the thermos, of course, all bets are off. There
will be a sudden influx of new oxygen; during the pouring process a
larger surface area will be exposed to the air; and a much greater
pool of oxygen exists within the thermos after you've opened it. Once
you've cracked the seal on your thermos, you want to drink the coffee
pretty quickly.
So what to do?
Well, you can assist your cause in a few ways. Obviously, a starting
point is to use the best coffee, filtered water, and the cleanest
equipment (putting the world's best coffee into an old, stained,
thermos won't get you very far...). Second, fill your thermos as
close to the stopper as you can get it. The less air, the less
degradation. Third, drink your coffee as soon as possible (kinda
defeats the purpose, I know, but there you have it). In other words,
rather than making one large pot to last through the day, maybe make
two or three smaller pots. Which brings us to my fourth point...use
the smallest thermos that's practical. Use more than one, if
necessary.
There are many reasons for that. One is that the smaller your
thermos, the less surface area is exposed to air. A second is that
you'll empty the thermos relatively quickly, leaving less mediocre
coffee at the bottom of the flask. By dividing your coffee among two
or three smaller thermoses, you will notice a distinct improvement.
My wife's "take to class" thermos is very slender (reduced surface
area)and holds only slightly more than a large coffee mug. We've had
very good results with that.
Finally, for larger quantities of coffee (making ahead for
entertaining, for example) you may want to invest in a pump-style
vacuum carafe. These are widely-used in today's foodservice industry;
their design minimizes the quantity of air reaching the coffee.
Search Strategy
Most of the information above was accumulated in the course of twenty
years' addiction to coffee (beans of life!); a career in sales will do
that to you. I'm in the course of a career change from sales to
cooking; I used my textbook (On Cooking: A Textbook of Culinary
Fundamentals, Labensky et al, Prentice-Hall 1999) in the preparation
of this answer.
Any Google search on coffee-related keywords will return thousands and
thousands of sites, most of them enthusiast sites or boastful "about
us" pages from coffeeshops. These will contain huge amounts of
information, frequently contradictory (or even self-contradictory).
For your amusement, I include a link to one page which has summarized
a large quantity of this information for purposes of humour:
http://www.baetzler.de/humor/alt_coffee_guide.html
The wonderful people who cater to my own personal addiction are here
(mine is the organic, fairly-traded, dark-roasted Central American
blend...mmmmm):
http://www.javablendcoffee.com/
Thank you for your question, Deerob, fuelled by frustration though it
was. I could talk about food and drink all day.... :)
-Chromedome

deerob-ga
rated this answer:
and gave an additional tip of:
$1.00

Wow - great answer! I am particularly impressed with the thuroughness
of this response given the meager pricing of my question. Truly
excellent. Thanks!
(with this i am one step further in my journey towards coffee-geekdom
like Chromedome ;-)

Thanks for your kind words and high rating, Deerob. Yeah, it's
amazing how much more work a person'll do for a question they've got a
personal interest in. A prime example would be a question I answered
earlier about the Halifax Explosion...an event which took place
literally just outside where my living room window is, today.
I'll throw one more link your way, courtesy of our beloved
Pinkfreud-ga. This goes seriously into the chemistry of coffee
bitterness; the links at the bottom cover acidity and aroma.
http://www.coffeeresearch.org/science/bittermain.htm
-Chromedome

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